JP4642213B2 - Image forming apparatus - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
  The present invention relates to an image forming apparatus employing an image forming process such as an electrophotographic system or an electrostatic recording system.About. More detailsThermal fixing process using the unfixed toner image of the target image information formed and supported on the recording material (transfer material, printing paper, photosensitive paper, electrostatic recording paper, etc.) in the image forming process section as a fixed image YouRufuFilm heating system fixingForming apparatus having meansAbout.
[0002]
[Prior art]
  Conventionally, a heat roller type or a film heating type device has been widely used as a heat fixing device in an image forming apparatus.
[0003]
  In particular, a system that suppresses power consumption as much as possible without supplying power to the heat-fixing device during standby, and more specifically, a film heating system that fixes a toner image on a recording material through a film between a heater section and a pressure roller Japanese Patent Application Laid-Open Nos. 63-313182, 2-157878, 4-44075, 4-204980 and the like have been proposed and put to practical use.
[0004]
  Basically, as shown in a model diagram of the main part in FIG. 11, a heater (heating member, heating body) 31 fixedly supported on a stay holder (supporting body) 32, and a heat-resistant thin wall on the heater 31 An elastic pressure roller 40 is formed in which a fixing nip portion n having a predetermined nip width is formed with a film (hereinafter referred to as a fixing film) 33 sandwiched therebetween. The heater 31, stay holder 32, and fixing film 33 side is a fixing member (heating device) 30, and the elastic pressure roller 40 is a pressure member.
[0005]
  As the heater 31, a ceramic heater is generally used, and the heater 31 is heated and adjusted to a predetermined temperature by energization control to the energization heat generating resistance layer of the heater.
[0006]
  The fixing film 33 is a cylindrical or endless belt that is conveyed and moved in the direction of the arrow while being in close contact with and sliding on the surface of the heater 31 at the fixing nip n by the driving force (not shown) or the rotational force of the pressure roller 40. Or it is a roll-shaped end-web-shaped member.
[0007]
  In a state where the heater 31 is heated and adjusted to a predetermined temperature, and the fixing film 33 is conveyed and moved in the direction of the arrow, the material as a material to be heated is fixed between the fixing film 33 and the pressure roller 40 in the fixing nip n. When the recording material P on which the unfixed toner image t is formed and supported is introduced, the recording material P comes into close contact with the surface of the fixing film 33 and is nipped and conveyed together with the fixing film 33 through the fixing nip n. At the fixing nip n, the recording material P / toner image t is heated by the heater 31 via the fixing film 33, and the toner image t on the recording material P is heated and fixed. The recording material portion that has passed through the fixing nip n is peeled off from the surface of the fixing film 33 and conveyed.
[0008]
  As heater 31ofThe ceramic heater, for example, has a substrate length (perpendicular to the drawing) on one surface (the surface facing the fixing film 33) of a ceramic substrate (heater substrate) having electrical insulation, good thermal conductivity and low heat capacity such as alumina. Direction), silver paradium (Ag / Pd), Ta₂An energization heating resistance layer such as N is formed by screen printing or the like, and the heating resistance layer forming surface is covered with a thin glass protective layer. The ceramic heater 31 is energized from a power supply device (not shown) to the energized heat generating resistance layer through the power supply unit, so that the energized heat generating resistance layer generates heat and the entire heater including the ceramic substrate and the glass protective layer is rapidly heated. To do.
[0009]
  The temperature rise of the heater 31 is detected by a temperature detection means 34 installed in an area where all the recording materials that can be conveyed on the back surface of the heater are conveyed, and fed back to an energization control unit (not shown). The energization control unit controls power supply to the energization heating resistor layer so that the heater temperature detected by the temperature detection unit 34 is maintained at a predetermined substantially constant temperature (fixing temperature).
[0010]
  As a control method, there are a wave number control method for controlling increase / decrease in the wave number of an applied voltage, a phase control method for applying a voltage from each phase angle of the voltage, and the like. By these control methods, the heater 31 is heated and adjusted to a predetermined fixing temperature.
[0011]
  In such a film heating type heat fixing apparatus, the pressure roller 40 having an elastic layer is provided on the flat lower surface of the heater 31 to which the elastic roller is pressed because of the high rigidity of the ceramic heater 31 as a heating member. Thus, the fixing nip portion n having a predetermined width is formed flat at the pressure contact portion, and only the fixing nip portion n is heated, thereby realizing quick start heating and fixing.
[0012]
  As described above, the conventional film heating type heat fixing device satisfies the quick start property (on-demand property) by suppressing the heat capacity of the fixing member 30 as small as possible. Is set finely. One example of the temperature control method of the heater 31 is given below. In the film heating type heat fixing device, the temperature adjustment temperature of the heater 31 as the heating member is changed by the surface temperature of the pressure member such as the pressure roller 40.
[0013]
  That is, when the energization of the energization heat generation resistance layer of the heater 31 is not performed for a long time (hereinafter referred to as a stamped state), the pressure member 40 is sufficiently cooled, and in this case In order to heat-fix the unfixed toner image t on the recording material P, the amount of heat given from the fixing member 30 is mainly used. On the other hand, the surface of the pressure member 40 is sufficiently heated after the heat fixing operation continues for a while. In such a case, not only the amount of heat from the fixing member 30 but also the pressure member 40 is heated. The amount of heat from the toner can also be used as the amount of heat necessary for heating and fixing the unfixed toner image t onto the recording material P. For this reason, the amount of heat applied from the fixing member 30 to the recording material P can be reduced compared with the heat fixing from the state where the pressure member 40 is cooled. On the other hand, if it is not kept small, an excessive amount of heat is applied to the toner image t on the recording material P, which causes problems such as high temperature offset. Therefore, the temperature adjustment temperature of the heater 31 in the fixing member 30 needs to be sequentially changed according to the temperature rising state of the pressure member 40.
[0014]
  An example of this temperature control is shown in FIG. In FIG. 12, the horizontal axis represents the number of sheets passed from the state where the pressure member is cooled, and the vertical axis represents the control temperature of the heater 31. In the case where an unfixed image is formed by the image forming apparatus (image forming process unit) when the pressure member 40 is cooled, the temperature detection means at a temperature adjustment temperature of 210 ° C. corresponding to the first sheet in the figure. The temperature of the heater 31 is controlled to be kept constant by temperature control means including 34. When the unfixed screen is continuously heat-fixed, the temperature of the pressure member surface gradually increases according to the number of fixed sheets. Therefore, in FIG. 12, the temperature of the heater 31 is adjusted to 5 ° C. for every 20 sheets.One by oneThe temperature is lowered to 185 ° C.
[0015]
  Further, when the heat fixing is started after the pressure member 40 has been warmed to some extent, the control temperature of the heater 31 is set according to the temperature detected by the temperature detecting means 34 when the print signal is input to the image forming apparatus. decide. In this method, the heat capacity of the fixing member 30 such as the heater 31 and the film 33 is very small, and the energization of the energization heat generation resistance layer of the heater 31 is shut down during standby, so that the heat capacity is higher than that of the fixing member 30. This is possible because the temperature detecting means 34 converges to the temperature of the large pressure member 40. For example, when the detection temperature of the temperature detection means 34 at the start of printing (before the start of energization to the energization heating resistor layer) is 50 ° C. to 70 ° C., the detection temperature starts from 205 ° C. corresponding to the 21st sheet in FIG. When the temperature is 70 ° C. to 90 ° C., temperature control is started from 200 ° C. corresponding to the 41st sheet. Thereby, optimal heater temperature control becomes possible according to the heating condition of the pressure member 40.
[0016]
  However, the detection of the surface temperature of the pressure member 40 by the temperature detection means 34 is not accurate immediately after heat fixing. That is, the temperature detecting means 34 formed at a position corresponding to the fixing nip portion n and the pressure member 40 outside the fixing nip portion are compared with a part of the pressure member 40 corresponding to the stopped fixing nip portion n. The surface is exposed to the airRutaTherefore, it is easy to be cooled. For this reason, the temperature detected by the temperature detecting means 34 is very high compared to the surface of the pressure member 40 outside the fixing nip within a certain time after the heat fixing. For this reason, it is difficult to determine the heater temperature control temperature at the time of the next printing within a predetermined time after the end of the heat fixing operation, in order to prevent both fixing failure and high temperature offset. It is possible to perform more optimal temperature control by determining the heater temperature adjustment temperature at the next printing from the heater temperature adjustment temperature at the previous printing.
[0017]
  Therefore, the conventional method has two heater temperature control tables when the recording material is conveyed and fixed continuously and when the recording material is conveyed intermittently. This is because when the recording material is conveyed intermittently, the temperature rise on the surface of the pressure member 40 is larger than in the case of continuous conveyance. Therefore, it is necessary to change the optimum heater control temperature. For example, when temperature control of the heater 31 is controlled as shown in FIG. 12 during continuous printing, the heater temperature control as shown in FIG. 13 is performed during intermittent printing (when subsequent printing is started within a predetermined time). Implement control. That is, the heater control temperature is gradually lowered with a smaller number of sheets than during continuous printing. In FIG. 12, the heater temperature adjustment temperature is decreased by 5 ° C. for every 20 sheets during continuous operation, whereas the heater temperature adjustment temperature is decreased by 5 ° C. for every 5 sheets during intermittent printing in FIG. As a result, even when the pressure member 40 is excessively heated during intermittent printing, it is possible to perform optimum heat fixing without causing a high temperature offset.
[0018]
  When continuous printing and intermittent printing are mixed, heater temperature control can be performed using each table. For example, when continuous printing is started from a state where the pressure member 40 is sufficiently cooled and the transition from the 25th sheet to the intermittent printing is performed, the heater is controlled from the control corresponding to the seventh sheet at the time of intermittent printing in FIG. For example, temperature control is started. That is, control is performed by converting four continuous prints into one equivalent to intermittent print.
[0019]
  As described above, the conventional film heating type heat fixing apparatus achieves both sufficient fixing performance and high temperature offset prevention by performing fine heater temperature control as described above.
[0020]
[Problems to be solved by the invention]
  However, in the above-described film heating type heat fixing device, the heating (overheating) state of the pressure member varies depending on the intermittent time during intermittent printing, and fixing failure and high temperature offset may occur depending on the intermittent time. In particular, in the conventional example in which the heater temperature adjustment temperature decrease rate is constant under the same conditions regardless of the intermittent time, it is difficult to satisfy the fixing performance and prevent the high temperature offset.
[0021]
  In the above-described conventional example, the recording material conveyance speed can be handled up to a certain range. However, due to the speedup of the image forming apparatus, the margin of the amount of heat necessary for fixing and the amount of heat that does not cause a high temperature offset is reduced. In this case, it becomes more difficult to prevent both the fixing failure and the high temperature offset, and further, the cooling rate of the pressure member 40 after the energization of the energization heating resistor layer of the heater is shut down due to the influence of the outside air temperature. If they are different, the same heater temperature control can no longer be achieved.
[0022]
  Of the above problems, the speeding up of the image forming apparatus will be described. When an unfixed image is heat-fixed on a recording material, the temperature and heating time of a fixing member necessary for heat-fixing are greatly affected. In general, when the heating time can be increased (when the fixing nip width is increased or when the recording material conveyance speed of the image forming apparatus is low), the temperature of the fixing member can be generally lowered. This is because the amount of heat applied to the recording material is determined by the product of the temperature of the heating member and the time for which the recording material passes through the fixing nip. Therefore, in the case of a low-speed image forming apparatus, it takes a long time for heat fixing, so that the set temperature of the fixing member can be lowered. This widens the range of the optimum temperature control temperature that can be set for fixing performance and high temperature offset prevention.
[0023]
  On the other hand, when the conveyance speed of the image forming apparatus is increased, the range of the optimum temperature control temperature that can be set is narrowed, and it is difficult to ensure both fixing performance and prevention of high temperature offset. In order to avoid this problem, in order to increase the heating time, it is necessary to increase the width of the fixing nip, leading to an increase in the size of the apparatus, such as increasing the outer diameter of the pressure roller as the pressure member. End up.
[0024]
  In addition, as another problem, there is an influence of the outside air temperature, the lower the outside air temperature, the larger the temperature drop of the pressurizing member, and when uniform heater temperature control is performed according to each environment In addition, the heater control temperature range that achieves both the fixing performance and the prevention of high temperature offset becomes narrow. Therefore, it has been very difficult to prevent high temperature offset while satisfying the fixing performance.
[0025]
  In addition, the fact that the temperature of the recording material varies depending on the outside air temperature also affects the fixing performance and the high temperature offset, and considering these factors, the optimum heater control temperature range becomes very narrow. That is, when the temperature of the recording material is low, a heater temperature of a predetermined value or more is necessary to satisfy the fixing performance, whereas when the temperature of the recording material is high, a temperature of a predetermined value or less is required to prevent high temperature offset. It is necessary to keep it at the heater temperature. For this reason, when the speed of the image forming apparatus is increased, it is difficult to satisfy the fixing performance and prevent the high temperature offset by the conventional heater temperature control.
[0026]
  The present invention has been proposed in view of the above, and a film heating type heat fixing device (on-demand fixing device).Image forming apparatus provided withIn order to ensure sufficient fixing performance even during intermittent printing, and to prevent problems caused by overheating such as high temperature offsets, in particular, to perform optimum temperature control for speeding up the image forming apparatus, etc. To do.
[0027]
[Means for Solving the Problems]
  The present invention is characterized by the following configuration.Image forming apparatusIt is.
[0028]
  (1)A fixing film, a heater in contact with the fixing film, a pressure roller that forms a fixing nip portion with the heater via the fixing film, and a temperature detection element that detects the temperature of the heater, Fixing means for heating and fixing the toner image on the recording material to the recording material while nipping and conveying the recording material at the fixing nip portion;
  Control means for controlling the power supplied to the heater so that the detection temperature of the temperature detection element maintains the control temperature, and is set such that the control temperature decreases as the number of continuous prints increases. Control means for storing a control temperature table;
  A counter that counts the number of prints,
In the image forming apparatus for setting the control temperature based on the control temperature table according to the count number of the counter,
  If the elapsed time from the end of the previous printing process and stopping the power supply to the heater to the time of receiving the current print signal is longer than a predetermined time, the initial value of the control temperature at the time of the current print is In accordance with the detection temperature of the temperature detection element at the time of receiving the print signal this time, the detection temperature is set to be higher as it is lower,
  When the elapsed time is within the predetermined time, the initial value of the control temperature at the time of the current printing is a count obtained by adding the final counted number at the time of the previous printing and the larger counted number at the shorter elapsed time. An image forming apparatus, wherein a control temperature corresponding to the number of sheets is set based on the control temperature table.
[0029]
  (2)A fixing film, a heater in contact with the fixing film, a pressure roller that forms a fixing nip portion with the heater via the fixing film, and a temperature detection element that detects the temperature of the heater, Fixing means for heating and fixing the toner image on the recording material to the recording material while nipping and conveying the recording material at the fixing nip portion;
  Control means for controlling the power supplied to the heater so that the detection temperature of the temperature detection element maintains the control temperature, and is set such that the control temperature decreases as the number of continuous prints increases. Control means for storing a control temperature table;
  A counter that counts the number of prints,
In the image forming apparatus for setting the control temperature based on the control temperature table according to the count number of the counter,
  It has an environmental temperature detection element that detects the environmental temperature where the device is installed,
  If the elapsed time from the end of the previous printing process and stopping the power supply to the heater to the time of receiving the current print signal is longer than a predetermined time, the initial value of the control temperature at the time of the current print is In accordance with the detection temperature of the temperature detection element at the time of receiving the print signal this time, the detection temperature is set to be higher as it is lower,
  When the elapsed time is within the predetermined time, the initial value of the control temperature at the time of the current printing is set to a final number of counts at the time of the previous printing, and a larger number of counts as the detected temperature of the environmental temperature detection element is higher. An image forming apparatus characterized in that a control temperature corresponding to the counted number of sheets is set based on the control temperature table.
[0030]
  (3)A fixing film, a heater in contact with the fixing film, a pressure roller that forms a fixing nip portion with the heater via the fixing film, and a temperature detection element that detects the temperature of the heater, Fixing means for heating and fixing the toner image on the recording material to the recording material while nipping and conveying the recording material at the fixing nip portion;
  Control means for controlling the power supplied to the heater so that the detection temperature of the temperature detection element maintains the control temperature, and is set such that the control temperature decreases as the number of continuous prints increases. Control means for storing a control temperature table;
  A counter that counts the number of prints,
In the image forming apparatus for setting the control temperature based on the control temperature table according to the count number of the counter,
  If the elapsed time from the end of the previous printing process and stopping the power supply to the heater to the time of receiving the current print signal is longer than a predetermined time, the initial value of the control temperature at the time of the current print is In accordance with the detection temperature of the temperature detection element at the time of receiving the print signal this time, the detection temperature is set to be higher as it is lower,
  When the elapsed time is within the predetermined time, the initial value of the control temperature at the time of the current printing, the final count number at the time of the previous printing, and the previous printing process are finished and the power supply to the heater is stopped. The control temperature is set to a control temperature corresponding to a count number obtained by adding a count number corresponding to a temperature decrease amount detected by the temperature detection element within a predetermined period immediately after the temperature detection element and adding a larger count number as the temperature decrease amount is smaller. An image forming apparatus that is set based on a table.
[0031]
[0032]
[0033]
[0034]
[0035]
[0036]
[0037]
[0038]
[0039]
[0040]
DETAILED DESCRIPTION OF THE INVENTION
  <First embodiment>
  (1) Example of image forming apparatus
  FIG. 1 is a schematic structural model of an example of an image forming apparatus according to the present invention. The image forming apparatus of this example is a laser beam printer using a transfer type electrophotographic process.
[0041]
  Reference numeral 1 denotes a photosensitive drum as an image carrier, which is formed by forming a photosensitive material such as OPC, amorphous Se, or amorphous Si on a cylindrical substrate such as aluminum or nickel.
[0042]
  The photosensitive drum 1 is rotationally driven in a clockwise direction indicated by an arrow with a predetermined peripheral speed, and the surface thereof is uniformly charged by a charging roller 2 as a charging device.
[0043]
  Next, the image exposure L by the laser scanner 3 which is an image exposure apparatus is received. The laser scanner 3 outputs a laser beam on / off-modulated in response to the time-series electric digital pixel signal of the image information, and performs scanning exposure L on the charging processing surface of the rotating photosensitive drum 1. By this scanning exposure, an electrostatic latent image of image information is formed on the photosensitive drum 1.
[0044]
  The electrostatic latent image is developed as a toner image by the developing device 4 and visualized. As a developing method, a jumping developing method or the like is used, and it is often used in combination with image exposure and reversal development.
[0045]
  On the other hand, a recording material (transfer material) P is fed to one sheet from a sheet feeding mechanism (not shown), and a predetermined control timing is applied to a transfer nip portion that is a pressure contact portion between the photosensitive drum 1 and a transfer roller 5 as a transfer device. , And is nipped and conveyed between the photosensitive drum 1 and the transfer roller 5 with a constant pressure. In the transfer nip portion, the toner image on the photosensitive drum 1 is transferred to the recording material P by the action of a transfer bias by a power source (not shown).
[0046]
  The recording material P that has passed through the transfer nip is separated from the surface of the photosensitive drum 1, holds the toner image, is conveyed to the fixing device 6, and is heated and pressurized at the fixing nip of the fixing device 6 to record the toner image. It is fixed on the material P to form a fixed image, and is discharged outside the apparatus.
  That is, the fixing device 6 passes the recording material introduced from the image forming process unit as described above, on which the unfixed image is formed, between the fixing nip formed by pressing the fixing member and the pressing member. Thus, the heat fixing device fixes the unfixed image on the recording material.
[0047]
  On the other hand, the transfer photosensitive toner remaining on the surface of the rotary photosensitive drum 1 after separation of the recording material is removed by a cleaning device (cleaner) 7 and is repeatedly used for image formation.
[0048]
  Reference numeral 8 denotes a cooling fan provided in the image forming apparatus, and reference numeral 9 denotes an outside air temperature detecting means provided in the vicinity of the cooling fan 8.
[0049]
  (2) Fixing device 6
  FIG. 2 is an enlarged cross-sectional model view of the fixing device 6. The fixing device 6 of this example is a film heating method and a rotating body for pressurization using a cylindrical (endless belt-shaped) fixing film disclosed in Japanese Patent Application Laid-Open Nos. 4-44075 to 44083 and 4-20980 to 204984. This is a driving method (tensionless type) fixing device.
[0050]
  Reference numeral 10 denotes a fixing member, and reference numeral 20 denotes a pressure roller as a pressure member, and a fixing nip portion n is formed by the pressure contact between both 10 and 20.
[0051]
  The fixing member 10 is a member having a longitudinal direction in the drawing as a longitudinal direction. The heat insulating stay holder 12 having a heat resistance and rigidity having a substantially semicircular arc shape in cross section is disposed on the lower surface of the heat insulating stay holder 12. A cylindrical (heat-resistant) heater 11 (heated body) 11 that generates heat when energized, and a heat-insulating stay holder 12 to which the heater 11 is attached. The fixing film 13 or the like.
[0052]
  The pressure roller 20 includes a cored bar 21 and an elastic layer composed of a heat-resistant rubber such as silicon rubber or fluorine rubber or silicon rubber formed by foaming silicon rubber, which is formed concentrically on the cored bar. It is the rotary body which consists of. On the elastic layer 22, a heat-resistant release layer 23 made of a fluororesin such as PFA, PTFE, FEP or the like may be formed.
[0053]
  The pressure roller 20 is arranged such that both ends of the cored bar 21 are rotatably supported by bearings between side plates on the front side and the back side of the apparatus chassis (not shown) via a bearing member.
[0054]
  The fixing member 10 is disposed on the upper side of the pressure roller 20 in parallel with the pressure roller 20 with the heater 11 facing downward, and both ends of the heat insulating stay holder 12 are pressure urging members such as springs (not shown). By energizing in the axial direction of the pressure roller 20, the downward surface of the heater 11 is applied to the elastic layer 22 of the pressure roller 20 via the fixing film 13 against the elasticity of the elastic layer. And a fixing nip portion n having a predetermined width is formed. The pressure roller 20 side can be pushed up and biased to the lower surface of the fixing member 10 by a pressure biasing member to form a fixing nip portion n having a predetermined width.
[0055]
  The pressure roller 20 is rotationally driven by the driving means M in a counterclockwise direction indicated by an arrow at a predetermined peripheral speed. A rotational force is applied to the cylindrical fixing film 13 by the pressure frictional force at the fixing nip n between the outer surface of the pressure roller 20 and the fixing film 13 by the rotational driving of the pressure roller 20, and the fixing film 13 is moved. The inner surface of the heat-insulating stay holder 12 is driven to rotate in the clockwise direction indicated by the arrow while sliding with the inner surface in close contact with the downward surface of the heater 11.
[0056]
  The pressure roller 20 is rotationally driven, and accordingly, the cylindrical fixing film 13 is driven and rotated, and the heater 11 is energized, the heater is heated to a predetermined temperature, and the temperature is adjusted. , The recording material P carrying the unfixed toner image t is introduced between the fixing film 13 and the pressure roller 20 in the fixing nip n, and the toner image carrying surface side of the recording material P is the fixing film in the fixing nip n. The fixing nip n is nipped and conveyed together with the film 13 in close contact with the outer surface of the film 13. In this nipping and conveying process, the heat of the heater 11 is applied to the recording material P via the fixing film 13, and the unfixed toner image t on the recording material P is heated and pressurized on the paper P to be melted and fixed. The recording material P that has passed through the fixing nip n is separated from the fixing film 13 by curvature.
[0057]
  a) Heater 11
  FIG. 3 is an enlarged cross-sectional model view of the fixing nip portion n. 4A is a partially cutaway plan view of the heater front side, and FIG. 4B is a plan view of the heater back side.
[0058]
  The heater 11 in this example is a so-called ceramic heater. Various configurations are known. A typical configuration is Al₂O₃(Alumina) / AlN (aluminum nitride), etc. on a ceramic substrate is printed and fired with a paste of a conductive heating resistor such as Ag / Pd to form a heating element, and further protect the heating element and ensure insulation. This is a long and planar member having a low heat capacity and formed with a glass coating layer for the purpose, and the temperature is rapidly raised as a whole by energizing the heating element.
[0059]
  Reference numeral 11a denotes a highly insulating ceramic substrate (heater substrate) such as alumina, which is a horizontally long and thin plate member having a direction perpendicular to the sheet passing direction as a longitudinal direction.
[0060]
  Reference numeral 11b denotes an energization heating resistor layer formed on the surface of the ceramic substrate 11a along the length of the substrate. Silver paradium (Ag / Pd) / RuO₂・ Ta₂It is provided by forming a pattern in a linear or narrow strip shape having a thickness of about 10 μm and a width of about 1 to 5 mm by screen printing or the like using an energization heating resistor paste such as N, and firing.
[0061]
  Reference numerals 11c and 11c denote power supply electrode portions, which are formed of Ag / Pt (silver / white silver) on the surfaces on both ends of the ceramic substrate 11a, respectively. The conductive heating resistor layer 11b is electrically connected to one end side and the other end side thereof through Ag and Ag / Pd conducting portions 11d and 11d. Each of the power supply electrode portion 11c and the conductive portion 11d is provided by forming a pattern by screen printing or the like using a conductive paste and baking it.
[0062]
  Reference numeral 11e denotes a protective layer that covers the surface side of the ceramic substrate 11a provided with the energization heating resistor layer 11b. The protective layer 11e is a thin heat-resistant glass coat, fluororesin, or the like that can withstand sliding with the fixing film 13.
[0063]
  At least one temperature detection means 14 such as a thermistor for detecting the temperature (heater temperature) of the ceramic substrate 11a raised in accordance with the heat generation of the energization heating resistor layer 11b is printed or adhered to the back surface of the ceramic substrate 11a. It is arranged.
[0064]
  The heater 11 is fixed to the lower surface of the heat insulating stay holder 12 with the heater substrate 11a on which the energized heat generating resistance layer 11b and the protective layer 11e are formed as the heater surface side and the heater surface side as the outside. The inner surface of the fixing film 13 is in close contact with the heater surface and slides.
[0065]
  21 is a control circuit (control CPU), 22 is an AC driver, and 23 is a commercial AC power source. The power supply electrode portions 11c and 11c of the heater 11 and the AC driver 22 are electrically connected via a connector (not shown). The AC driver 22 is controlled by the control circuit 21. Electrical temperature detection information of the thermistor 14 serving as a temperature detection element that detects the temperature of the heater is input to the control circuit 21 via the A / D converter 24.
[0066]
  Detection information of the outside air temperature detecting means 9 provided in the vicinity of the cooling fan 8 (FIG. 1) is also input to the control circuit 21 via the A / D converter 25.
[0067]
  The control circuit 21 controls the AC driver 22 to energize the energization heat generating resistance layer 11b of the heater 11 to generate heat.
[0068]
  By energizing the energization heat generating resistance layer 11b of the heater 11 as described above, the heater 11 is quickly heated by the heat generated by the energization heat generation resistance layer 11b. The temperature information of the heater 11 is input from the thermistor 14 to the control circuit 21 via the A / D converter 24. The control circuit 21 controls the AC driver 22 to control the heater temperature to a predetermined constant temperature based on the input temperature information. In other words, the heater temperature is controlled to a predetermined fixing temperature by supplying a power-controlled AC current to the energization heating resistor layer 11b of the heater 11. Alternatively, by appropriately controlling the voltage applied to the energization heating resistor layer 11b, the temperature necessary for fixing the toner image on the recording material can be maintained while maintaining the temperature control temperature in the fixing nip n substantially constant. Do.
[0069]
  As the control method, a wave number control method for controlling input power by the wave number of the AC voltage, a phase control method for energizing to the next zero cross after a predetermined delay time from the zero cross of the AC voltage, or the like is applied.
  As described above, the heater 11 can be controlled to a predetermined temperature by the control circuit 21 (temperature control means) including the thermistor 14 (temperature detection means).
[0070]
  b) Insulated stay holder 12
  The heat insulating stay holder 12 holds the heater 11 and prevents heat radiation in the direction opposite to the fixing nip. The heat insulating stay holder 12 is formed of liquid crystal polymer, phenol resin, PPS, PEEK, etc., and the fixing film 13 is loosely removed. It is fitted.
[0071]
  Further, since the fixing film 13 rotates while rubbing against the internal heater 11 and the heat insulating stay holder 12, it is necessary to suppress the frictional resistance between the heater 11 and the heat insulating stay holder 12 and the fixing film 13. For this reason, a small amount of lubricant such as heat-resistant grease is interposed on the surfaces of the heater 11 and the heat insulating stay holder 12. Thereby, the fixing film 13 can be smoothly rotated.
[0072]
  c) Fixing film 13
  The fixing film 13 is a film having a small heat capacity and has a thickness of 100 μm or less to enable quick start, such as polyimide, polyamideimide, PEEK, PES, PPS, PFA, PTFE, FEP having heat resistance and thermoplasticity. It is a resin film or a metal film such as SUS, Al, Ni. Further, the film needs to have a thickness of 20 μm or more as a film having sufficient strength and excellent durability for constituting a long-life heat fixing device. Therefore, the thickness of the fixing film 13 is optimally 20 μm or more and 100 μm or less.
[0073]
  Further, a conductive primer layer in which a conductive agent such as carbon black is dispersed is formed outside the base layer, and in order to prevent electrostatic offset due to charge-up of the fixing film 13, a grounded state or a rectifying element such as a diode It is in a grounded state via
[0074]
  Further, in order to prevent offset and to ensure the releasability of the recording material, the surface layer is mixed or singly coated with a heat-resistant resin having good releasability such as PFA, PTFE, FEP, and silicone resin.
[0075]
  (3) Temperature control temperature switching control, etc.
  In the case of the above configuration, the fixing member 10ofThe heater 11, the fixing film 13, the stay holder 12, etc. are made as low as possible in order to ensure quick start performance. Therefore, it is necessary to switch the temperature control temperature depending on the heating condition of the pressure roller 20.
[0076]
  In particular, when the speed of the image forming apparatus is increased, a finer temperature setting is required in order to obtain sufficient fixing performance and prevent problems due to overheating such as high temperature offset.
[0077]
  An example of setting the temperature control temperature is the same as that of the conventional example, and has a heater temperature control temperature table as shown in FIG.
[0078]
  IeFIG.When the pressure roller 20 is sufficiently close to the outside temperature of the image forming apparatus, heat fixing is performed from a heater temperature of 210 ° C. The temperature of the pressure roller 20 is detected from the temperature detected by the temperature detecting means 14 provided on the back surface of the heater 11 described above. This is a means that is possible because the present configuration does not energize the energization heating resistor layer 11b of the heater 11 in the standby state. However, it is almost impossible for the temperature detecting means 14 to detect the surface temperature of the pressure roller 20 immediately after heat fixing. This is because the temperature detection means 14 is in the fixing nip equivalent part and the cooling rate is different from the surface of the pressure roller 20 outside the fixing nip exposed to the outside air. It takes about 1 minute after the last heat fixing until the temperature detection means 14 on the back surface of the heater reaches a temperature close to the surface temperature of the pressure roller 20, preferably about 2 minutes after the power is turned off. . Therefore, the surface temperature of the pressure roller 20 can be detected according to the detected temperature of the temperature detecting means 14 after one minute after the recording material is once heated and fixed and the energization of the energization heat generating resistance layer 11b of the heater 11 is shut down. Therefore, it is possible to detect that the pressure roller surface is close to the outside air temperature as described above.
[0079]
  In addition, since it is possible to detect that the pressure roller is heated to some extent by the above method, the heater shown in the table below is used to control the heating heater according to the heating condition of the pressure roller. The temperature can be determined.
[0080]
  For example, when the detected temperature of the temperature detecting means 14 on the back surface of the heater immediately before printing is 50 ° C. to 70 ° C., temperature control is started from 205 ° C. corresponding to the 21st sheet in FIG. In the case of 70 ° C. to 90 ° C., temperature control is started from 200 ° C. corresponding to the 41st sheet (hereinafter referred to as sheet number control).
[0081]
  That is, the heater temperature control is performed by setting the apparent number of sheets to +20 or +40 according to the temperature detected by the temperature detecting means 14 at the start of temperature control. Table 1 shows the temperature control table.
[0082]
[Table 1]
[0083]
  The above shows the temperature control method during continuous printing, which is the same as the conventional example.
[0084]
  A control method related to the present embodiment during intermittent operation will be described below. A flowchart of this control method is shown in FIG. When intermittent, the surface temperature of the pressure member 20 becomes higher than when continuous. This is because the recording material is in the fixing nip.PinchingThis is because the energization time to the energization heat generating resistor layer 11b of the heater 11 other than when being transported becomes longer. That is, the time for which the surface of the pressure member 20 is heated by the heating of the heater 11 becomes longer. Therefore, if the temperature control temperature is lowered every 20 sheets as described above, the surface of the pressure member is excessively heated. Therefore, it is necessary to increase the apparent number of the number control according to the heating state of the pressure member surface temperature. For this reason, there is also a method shown in the conventional example, but when the conditions such as intermittent time and use environment temperature are different, the temperature control method is not in accordance with the heating state of the pressure member, and the optimum temperature control temperature In the case of a high-speed image forming apparatus in which the range is narrow, it is impossible to satisfy both of sufficient fixing performance and hot offset prevention.
[0085]
  For this reason, in this embodiment, in order to solve the above problem, optimal heater temperature control is performed according to the intermittent time.
[0086]
  In this embodiment, the time from when the energization to the energization heating resistor layer 11b of the heater 11 is shut down during intermittent printing until the next print signal is received (until the next image forming operation starts) is defined as the intermittent time. According to this time, the heater control temperature decreasing speed (temperature control temperature decreasing speed) is switched. That is, since the temperature detected by the temperature detecting means 14 on the back surface of the heater is close to the temperature of the pressure member 20 after a predetermined time from when the energization of the energization heating resistor layer b of the heater 11 is shut down during intermittent printing. As described above, the heater control temperature at the time of the next printing can be determined from the above table, but the energization of the energization heating resistor layer 11b of the heater 11 is shut down within 1 minute, more accurately. Since there is a large difference between the surface temperature of the pressure member 20 and the temperature detected by the temperature detecting means 14 disposed on the back of the heater during a printing operation within 2 minutes, the energization of the heater 11 is shut down or the power consumption is reduced. Since the apparent number of sheets in the above-mentioned number control method is changed according to the time from when the next print signal is received, the increase in the number of sheets is determined. It is possible to provide a more optimal heater temperature control temperature.
[0087]
  Normally, in the film heating method in which the energization heating resistor layer 11b of the heater 11 is not energized during standby, the surface temperature of the pressure member 20 becomes lower as the intermittent time is longer. Therefore, for example, as shown in the table of Table 2, the number of sheets for the number control is switched according to the intermittent time.
[0088]
[Table 2]
[0089]
  By determining the increment of the number of sheets at the time of the intermittent control in the temperature control of the number control method according to the intermittent time by the above method, an appropriate heater control temperature can be obtained according to the heating state of the pressure member 20.
[0090]
  As a result, even when the recording material conveyance speed of the image forming apparatus is increased, it is possible to easily cope with it, and it is possible to prevent problems such as high temperature offset while satisfying sufficient fixing performance.
[0091]
  In this embodiment, the temperature control temperature is determined based on the number of prints. However, the temperature control that gradually decreases the temperature control according to the number of prints (fixed number of recording materials) is at least a fixed number. It is possible to adapt to any type of temperature control used for (fixed number of fixed sheets).
[0092]
  <Second embodiment>
  In this embodiment, the image forming apparatusPlaceSince the configuration of the heat fixing device is the same as that shown in the first embodiment, the description thereof will be omitted.
[0093]
  In the present embodiment, a temperature detection unit different from the temperature detection unit 14 provided on the back surface of the heater is provided, and the temperature of the heater is controlled using the temperature detected by the temperature detection unit.
[0094]
  In this embodiment, for example, as shown in FIG. 1, an intake fan 8 is provided on the side surface of the image forming apparatus, and a temperature detecting means 9 is provided in the vicinity of the intake fan 8. When the image forming apparatus receives the print signal and starts the printing operation, the intake fan 8 sucks outside air into the main body of the image forming apparatus by a rotation operation, and thereby temperature detecting means provided near the intake fan 8. 9 detects a temperature close to the outside air temperature.
[0095]
  The intake fan 8 may have a purpose of cooling a temperature rising portion such as the photosensitive drum 1, the developing device 4, an unillustrated electrical board or scanner unit, and is an exhaust fan that exhausts air in the image forming apparatus. May be. However, it is better that the temperature detecting means 9 is disposed at a position where the outside air temperature can be accurately detected.
[0096]
  With the above configuration, the environmental temperature used by the image forming apparatus is controlled by the control circuit.21Therefore, more precise heater temperature control can be achieved by a combination of the detected temperature (outside temperature) and the detected temperature of the temperature detecting means 14 provided on the back surface of the heater. A flowchart of the control method of this embodiment is shown in FIG.
[0097]
  For example, the heater temperature adjustment control temperature is determined using the table in Table 3 and FIG. 7 according to the temperature detected by both the temperature detecting means 9 and 14 at the start of printing.
[0098]
  As shown in the table of Table 3, the temperature adjustment table used according to the temperature detected by the temperature detecting means 9 for detecting the outside air temperature immediately before starting the energization of the energization heating resistor layer 11b of the heater 11 at the start of printing. Choose. Further, in each table of FIG. 7, the temperature adjustment start position and the heater temperature control during continuous printing are the same as the methods shown in the first embodiment and the conventional example.
[0099]
  On the other hand, in this embodiment, the increase in the number of sheets is determined in order to determine the apparent number of sheets at the time of the intermittent operation in the sheet number control method shown in the first embodiment based on the temperature detected by the outside air temperature detecting means 9 during the intermittent operation.
[0100]
  For example, as shown in the table of Table 3, according to the detected outside air temperature, when the temperature is 20 ° C. or less, the increment of the number of sheets is set to 5; In the above case, it is 20 sheets. This is because the lower the outside air temperature during intermittent printing, the stronger the degree of exposure to cooling. Further, since the temperature of the recording material follows the outside air temperature, when the outside air temperature is low, the amount of heat necessary for heating and fixing the unfixed toner image increases.
[0101]
[Table 3]
[0102]
  As described above, according to the present embodiment, the rate of decrease in the heater temperature adjustment temperature during intermittent printing is changed using information on the outside air temperature. Specifically, the control is performed such that the higher the outside air temperature is, the lower the heater temperature adjustment temperature is set with a smaller number.
[0103]
  As a result, even when the use environment temperature of the image forming apparatus is different, it is possible to perform appropriate heater temperature control while preventing similar problems such as high temperature offset while satisfying similar fixing performance. In addition, the heater temperature can be controlled in consideration of the temperature of the recording material.
[0104]
  In this embodiment, the heater temperature control employs a method in which the temperature is lowered step by step depending on the number of sheets. However, other temperature controls for gradually lowering the temperature in accordance with the number of sheets as in the first embodiment. It may be a method.
[0105]
  Further, it is desirable that the temperature control method at the time of continuous printing is temperature control determined by the outside air temperature or the temperature detected by the temperature detection means on the back surface of the heater as shown in FIG.
[0106]
  <Third embodiment>
  In the present embodiment, the energization of the energization heating resistor layer 11b of the heater 11 immediately after the last recording material is heated and fixed is shut down or consumed depending on the heating state of the pressure member 20 and the use environment temperature. The temperature detected by the temperature detection means 14 on the back surface of the heater after the power is reduced is used.
[0107]
  In order to further improve the detection accuracy, it is desirable to shut down the energization of the energization heating resistor layer 11b of the heater 11 after the last recording material is heated and fixed. FIG. 8 shows a flowchart of the control method of this embodiment that uses the temperature detected by the temperature detecting means 14 on the back surface of the heater.
[0108]
  The detected temperature of the temperature detecting means 14 gradually decreases from the time when the heating of the heater 11 is finished. FIG. 9 shows the temperature drop. In FIG. 9, the horizontal axis indicates the current applied to the heating resistance layer.shutIndicates the time since the down. The minus sign indicates the time of energization just before shutting down. The vertical axis indicates the temperature detected by the temperature detection means 14. According to FIG. 9, it can be seen that the detected temperature gradually decreases immediately after the energization of the energization heating resistor layer 11 b of the heater 11 is shut down.
[0109]
  Moreover, it shows that the temperature drop rate of Comparative Example 2 is faster than the temperature drop of Comparative Example 1. This occurs due to differences in the heating state of the pressure member 20 and the operating environment temperature of the image forming apparatus.
[0110]
  Therefore, for each example, the apparent number is determined by changing the increase number in the temperature control of the number control method shown in the first and second embodiments. For example, as shown in the table of Table 4, the number of sheets is increased by the number control according to the amount of temperature decrease after 0.2 seconds from 0.8 seconds immediately after the energization of the energization heating resistor layer 11b of the heater 11 is shut down. Set.
[0111]
  That is, in the case of Comparative Example 1 where the lowering temperature is 24 ° C., it is indicated that the pressure member 20 is heated as compared with Comparative Example 2 or the operating environment temperature of the apparatus is high.
[0112]
  On the other hand, Comparative Example 2 shows that the temperature drop is 38 ° C., and the heating state of the pressure member 20 is weaker than that of Comparative Example 1, or the apparatus is used in a low temperature environment.
[0113]
  Therefore, in Comparative Example 1, in order to prevent high temperature offset due to overheating, the increment of the number of sheets is set to 10 sheets, and the temperature control temperature lowering timing is made earlier than 5 sheets of Comparative Example 2. Yes.
[0114]
  For example, printing is started from a state in which the heat fixing device is sufficiently close to the room temperature by the temperature control shown in FIG.TemporaryIn the case of stopping and starting the printing operation again (when resuming printing within 1 minute in which the temperature detected by the temperature detecting means cannot detect the accurate heating state of the pressure member), in Comparative Example 1, the number of sheets is 10 The temperature is controlled at 200 ° C. corresponding to the 43rd sheet by adding the increment. On the other hand, in Comparative Example 2, the temperature control is performed at 205 ° C. corresponding to the 38th sheet by adding the increment of 5 sheets.
[0115]
[Table 4]
[0116]
  As described above, in this embodiment, by performing finer heater temperature control in consideration of the heating state (intermittent time, the number of intermittent sheets, etc.) of the pressure member 20 and the use environment temperature of the image forming apparatus, more optimal temperature control is performed. The unfixed toner image on the recording material can be fixed by heating.
[0117]
  That is, the control temperature decrease rate of the heater is switched according to the temperature decrease rate of the temperature detection means 14 on the back surface of the heater immediately after the unfixed toner image on the last recording material is heated and fixed. In particular, when the temperature decrease rate is low, it is considered that the pressure member 20 is sufficiently heated. Therefore, the heater control temperature decrease rate is increased in order to prevent high temperature offset.
[0118]
  On the other hand, when the temperature decrease rate is high, it is considered that the pressure member has not been sufficiently heated yet, or the image forming apparatus is used in a low temperature environment. Slow down.
[0119]
  As described above, optimal temperature control according to the temperature rise state of the pressure member and the use environment temperature of the image forming apparatus is achieved.
[0120]
  In this embodiment, the heater temperature control by the number control method has been described as an example. However, as in the first and second embodiments, the heater temperature is gradually decreased according to the number of prints. Needless to say, the present invention can also be applied to a temperature control system that shuts down energization of the energization heating resistance layer of the heater or reduces power consumption after the recording material is heated and fixed.
[0121]
  In addition, the heater that further considers the temperature of the recording material by combining the intermittent time information shown in the first embodiment and the outside air temperature detection temperature by the outside air temperature detecting means shown in the second embodiment. Temperature control becomes possible.
[0122]
  An example is shown in Table 5. Note that the increment in the number of sheets in each embodiment is determined by the weight for the control.
[0123]
[Table 5]
[0124]
  As described above, by combining the first and second embodiments with the present embodiment, it becomes possible to perform finer heater temperature control at the time of intermittent printing according to the respective conditions, and to increase the speed of the image forming apparatus. On the other hand, it is possible to achieve heat fixing that prevents a harmful effect caused by overheating of the pressure member such as a high temperature offset while ensuring sufficient fixing performance.
[0125]
  <Others>
  1) Of course, the configuration of the ceramic heater 11 is not limited to that of the embodiment.
[0126]
  2) The heating fixing device of the film heating type is the pressure rotating body driving method in the embodiment, but the driving roller is provided on the inner peripheral surface of the endless fixing film and driven while applying tension to the film. It may be an apparatus, or may be an apparatus of a system in which a film is formed into a roll-wrapped end web and this is driven to travel.
[0127]
  3) The film heating type heat fixing device may be an electromagnetic induction heating type using an electromagnetic induction heating member such as an iron plate as a heater.
[0128]
  FIG. 10 is a schematic configuration model diagram of an example of a heating and fixing apparatus of a film heating method, an electromagnetic induction heating method, and a pressing rotary member driving method. 11A is an electromagnetic induction exothermic member such as an iron plate as a heater.50Is the excitation coil51And magnetic core52A magnetic field generator (magnetic field generating means). Excitation circuit53From exciting coil51The heater 11A generates heat by electromagnetic induction due to a high-frequency magnetic field (generated magnetic field) generated when a high-frequency current is supplied to the heater. The recording material P that is nipped and conveyed through the fixing nip n is heated by the heat of the heater 11A. The temperature information of the heater 11 </ b> A is taken into the control circuit 21 through the A / D converter 24 from the chip thermistor 14 as the heater temperature detecting means. The control circuit 21 is an excitation circuit for controlling the temperature of the heater 11A to a predetermined constant temperature based on the input temperature information.53To control. That is, an exciting coil that heats the heater 11A by electromagnetic induction.51By controlling the high-frequency current to the heater, the heater temperature is controlled to the target temperature (printing temperature).
[0129]
  Other heat fixing device configurations, controls, and the like are the same as those in the first to third embodiments.
[0130]
【The invention's effect】
  As described above, according to the present invention,If the elapsed time is longer than the predetermined time, the control temperature is set according to the detection temperature of the temperature detection element that detects the temperature of the heater. If the elapsed time is within the predetermined time, the control temperature is set by a different method. The temperature setting accuracy is improved.
[Brief description of the drawings]
FIG. 1 is a schematic configuration model diagram of an image forming apparatus according to an embodiment.
FIG. 2 is an enlarged model view of the heat fixing device.
FIG. 3 is an enlarged model view of a fixing nip portion.
[Fig. 4] Explanatory diagram of heater configuration
FIG. 5 is a control flowchart in the first embodiment.
FIG. 6 is a control flowchart in the second embodiment.
[Fig. 7] Temperature table of heater temperature control system
FIG. 8 is a control flowchart in the third embodiment.
FIG. 9 is a graph of detected temperature transition by temperature detecting means after heat fixing.
FIG. 10 is a schematic configuration model diagram of an example of an electromagnetic induction heating type heat fixing device;
FIG. 11 is a structural model diagram of the main part of a film heating type heat fixing device.
FIG. 12 is a temperature table for a heater temperature control method (part 1).
FIG. 13 is a temperature table for a heater temperature control method (part 2).
[Explanation of symbols]
  DESCRIPTION OF SYMBOLS 10 ... Fixing member, 11 ... Heater, 12 ... Heat insulation stay holder, 13 ... Fixing film, 14 ... Temperature detection means, 20 ... Pressure member (pressure roller), n ... Fixing nip, P ... Recording material

Claims (3)

A fixing film, a heater in contact with the fixing film, a pressure roller that forms a fixing nip portion with the heater via the fixing film, and a temperature detection element that detects the temperature of the heater, Fixing means for heating and fixing the toner image on the recording material to the recording material while nipping and conveying the recording material at the fixing nip portion;
Control means for controlling the power supplied to the heater so that the detected temperature of the temperature detecting element maintains the control temperature, and is set so that the control temperature decreases as the number of continuous prints increases. Control means for storing a control temperature table;
A counter that counts the number of prints,
In the image forming apparatus for setting the control temperature based on the control temperature table according to the count number of the counter,
If the elapsed time from the end of the previous printing process and stopping the power supply to the heater to the time of receiving the current print signal is longer than the predetermined time, the initial value of the control temperature at the time of the current print is In accordance with the detection temperature of the temperature detection element at the time of receiving the print signal this time, the detection temperature is set to be higher as it is lower,
When the elapsed time is within the predetermined time, the initial value of the control temperature at the time of the current printing is a count obtained by adding the final counted number at the time of the previous printing and the larger counted number as the elapsed time is shorter An image forming apparatus, wherein a control temperature corresponding to the number of sheets is set based on the control temperature table. A fixing film, a heater in contact with the fixing film, a pressure roller that forms a fixing nip portion with the heater via the fixing film, and a temperature detection element that detects the temperature of the heater, Fixing means for heating and fixing the toner image on the recording material to the recording material while nipping and conveying the recording material at the fixing nip portion;
Control means for controlling the power supplied to the heater so that the detection temperature of the temperature detection element maintains the control temperature, and is set such that the control temperature decreases as the number of continuous prints increases. Control means for storing a control temperature table;
A counter that counts the number of prints,
In the image forming apparatus for setting the control temperature based on the control temperature table according to the count number of the counter,
It has an environmental temperature detection element that detects the environmental temperature where the device is installed,
If the elapsed time from the end of the previous printing process and stopping the power supply to the heater to the time of receiving the current print signal is longer than the predetermined time, the initial value of the control temperature at the time of the current print is In accordance with the detection temperature of the temperature detection element at the time of receiving the print signal this time, the detection temperature is set to be higher as it is lower,
When the elapsed time is within the predetermined time, the initial value of the control temperature at the time of printing this time is the final count number at the time of the previous printing, An image forming apparatus characterized in that a control temperature corresponding to the counted number of sheets is set based on the control temperature table. A fixing film, a heater in contact with the fixing film, a pressure roller that forms a fixing nip portion with the heater via the fixing film, and a temperature detection element that detects the temperature of the heater, Fixing means for heating and fixing the toner image on the recording material to the recording material while nipping and conveying the recording material at the fixing nip portion;
Control means for controlling the power supplied to the heater so that the detected temperature of the temperature detecting element maintains the control temperature, and is set so that the control temperature decreases as the number of continuous prints increases. Control means for storing a control temperature table;
A counter that counts the number of prints,
In the image forming apparatus for setting the control temperature based on the control temperature table according to the count number of the counter,
If the elapsed time from the end of the previous printing process and stopping the power supply to the heater to the time of receiving the current print signal is longer than the predetermined time, the initial value of the control temperature at the time of the current print is In accordance with the detection temperature of the temperature detection element at the time of receiving the print signal this time, the detection temperature is set to be higher as it is lower,
When the elapsed time is within the predetermined time, the initial value of the control temperature at the time of the current printing, the final count number at the time of the previous printing, and the previous printing process are finished and the power supply to the heater is stopped. The control temperature is set to a control temperature corresponding to a count number obtained by adding a count number corresponding to a temperature decrease amount detected by the temperature detection element within a predetermined period immediately after the addition, and a larger count number as the temperature decrease amount is smaller. An image forming apparatus that is set based on a table.